Vacuum fluroescent devices are those in which phosphor coated anode segments are bombarded with low velocity electrons from a thermionically heated filament all contained within an evacuated envelope. The electrons are accelerated from the filament to the anodes by a small electric potential applied therebetween. Electric potentials on the order of a few volts to a few tens of volts are typically employed.
For reasons of economy, it is desired to use directly heated filaments rather than indirectly heated filaments. The voltage drop from one end of the filament to the other causes a difference in accelerating potential between the filament and the anodes from end to end of the filament. In multidigit vacuum fluorescent devices, a perceptible variation in digit brightness is seen due to the difference in potential applied between the filament and the anodes from end to end due to the filament drop.
U.S. Pat. No. 4,045,704 attempts to solve the problem of varying brightness by interposing control grids at varying spacings between the filament and the anodes. By placing the grids closer to the filament at the high-potential end of the filament and progressively further away towards the low-potential end, substantially uniform brightness of the phosphor is achieved.
In U.S. Pat. No. 4,049,993 a similar benefit is achieved by the installation of a filament which slopes downward from one end to the other thus being closer to the grids and/or anode at the positively charged end of the filament than at the negatively charged end. This achieves a uniform electric field between the filament and the respective anode segments along the length of the filament to achieve substantially uniform electron bombardment velocity. This patent also proposes combining varying grid heights with the sloping filament to further improve the brightness uniformity.
Each of the structures recited in the preceding requires special precision treatment of the internal structure of the vacuum fluorescent display device.